The present invention relates generally to a method of hard facing iron-based industrial components subjected to service conditions requiring good resistance to corrosion and wear such as rolls for steel mills and paper plants, ship and submarine main propulsion shafts, mining equipment, construction equipment, agricultural equipment, and pipes.
Weld surfacing of machine or industrial components to modify surface properties has been a common engineering practice. Submerged arc welding (SAW) is the most commonly used process for surfacing steel mill rolls; either wire or strip electrodes can be used for this purpose. Surfacing by SAW using strip electrodes was first employed in the early 1960s and has been extensively used in the industry since then as disclosed in Campbell, H. C. and Johnson, W. C. "Cladding and Overlay Welding with Strip Electrodes," Weld. J., Vol. 45(5), 1966, 99 399-409. However, SAW is an inherently high dilution process, requiring a multiple layer deposit. Dilution occurs when alloy constituents such as Cr and Ni which are typically found in significantly higher concentrations in an electrode alloy than in substrate alloys diffuse into the molten substrate. The result is disadvantageous loss of constituents into the substrate which constituents are intended for the overlayer. SAW is a high dilution process because the force of the arc penetrates deeper into the substrate thereby melting more of the substrate for the electrode alloy constituents to diffuse into. Also, the force of the arc tends to facilitate the diffusion of the electrode alloy constituents by mixing the molten substrate and electrode alloy together. Thus, because of the high dilution, surfacing operations using SAW usually require three or more layers to obtain the desired deposit chemistry, resulting in increased labor and material costs.
In the 1930s electroslag welding was developed by Hopkins in the United States. In 1971 a new process that combined electroslag welding characteristics with SAW strip surfacing was discovered. This was reported by Seidel, G. and Hess, H. in "Investigation of Electroslag Strip Cladding with Strip Electrodes," Schweissen und Schneiden, Vol. 23(10), 1971, pp 410-411. The new technique was eventually named electroslag surfacing (ESS) or electroslag strip cladding. Unlike electroslag welding which requires a mold or supporting surface, ESS operates in a flat or horizontal position with a rather shallow layer of molten slag. There is no arc during ESS except at the start of the process. Heat generated by ohmic heating of a molten slag pool melts filler metal (electrode), base metal (substrate) and flux to maintain a dynamically steady slag pool that helps to generate heat for the process and shields the molten metal from the atmosphere. The ESS process combines high deposition rates with low dilution, two mutually exclusive conditions in conventional arc welding processes. The uniform shallow penetration allows for more effective and easier alloy adjustment in the strip electrode to achieve the desired deposit chemistry, cleaner deposits, lower oxygen content and better slag detachment as reported by Gao, Y. P., Devletian, J. H. and Wood, W. E. in "Electroslag and Submerged Arc Cladding Ni Alloy 625," 73.sup.rd Annual AWS Convention, Chicago, Ill. 1992.
ESS with strip electrodes is widely used in Germany, Japan and to some extent in the United States. However, it is difficult to produce strip electrodes with certain high alloy compositions because, for example, the brittle nature or other properties of such compositions is not compatible with strip-forming processes. Thus, ESS with strip electrodes is limited to deposition of alloys which are readily formable into strip electrodes. Strip electrodes can be expensive and economically unattractive, or otherwise not feasible in certain applications.
The use of wires rather than strips in ESS is a candidate for overcoming the limitations of ESS with strips. However, the use of wires has its own limitations, such as difficulty in controlling free flow of a molten slag pool off of curved surfaces of substrates, and the fact that wire electrodes typically produce a non-uniform penetration profile which is much deeper where the axis of the wire intersects the substrate than at surrounding locations.
U.S. Pat. No. 4,000,010 discloses a process using consumable and non-consumable wire electrodes in electroslag surfacing of a roll. This process, however, is a relatively high dilution process because there is substantial contact between fresh substrate and molten slag prior to the substrate contacting molten metal being deposited. Further, the orientation of this process requires use of a cumbersome system employing a start tab and a dam to retain metal being deposited.